Display apparatuses have often to be adapted to display color adjustment. A typical display color adjustment includes adjustment of the color gamut. As known in the art, sRGB, AdobeRGB, NTSC (National Television System Committee) are typical display device specifications and these specifications individually specify the color gamut, that is, the chromaticity coordinates of the elementary color points (R, G and B) and the white point. A display apparatus is preferably adjusted to display the respective elementary color points and the white point with the chromaticity coordinates specified by the specifications supported by the display apparatus.
One known approach to achieve color adjustment is to perform digital processing on image data of images to be displayed.
One issue to be considered in color adjustment through digital processing is that a display apparatus usually has a non-linear input-output property. Such non-linear property is often referred to as gamma property. As is well known in the art, the gamma property of a display apparatus is represented by a gamma value γ in general. For a given gamma value γ, the output y of a display apparatus for an input x can be generally represented as the following function:Y=K·xγ,  (1)where K is a proportionality constant.
Accordingly, color adjustment through digital processing usually involves an operation on the basis of the gamma property of the display apparatus. One known approach is to perform a gamma conversion on image data, perform a color adjusting operation on the gamma-converted image data and then perform an inverse gamma conversion. For example, Japanese Patent Application Publication No. P2008-40305A discloses a color adjustment technique which involves serially performing: a gamma conversion, an RGB-XYZ conversion, an XYZ-LMS conversion, a color shade adjustment, an LMS-XYZ conversion and an inverse gamma conversion. Japanese Patent Application Publication No. P2008-141723A discloses a technique for converting YCbCr data into Adobe RGB data through an YCbCr-RGB conversion and an RGB-RGB conversion. This patent document discloses the RGB-RGB conversion involves a gamma conversion, a matrix operation, and an inverse gamma conversion. Japanese Patent Application Publication No. P2002-116750A discloses a technique for achieving a precise color correction with a simple circuit configuration. In the technique disclosed in this patent document, the color correction is achieved by serially performing a gamma conversion with an LUT (lookup table), a matrix operation, and an inverse gamma conversion with an LUT.
One issue is that a hardware circuit performing a gamma conversion and inverse-gamma conversion has an increased circuit size. The gamma conversion and inverse-gamma conversion include a power operation and a circuit to perform a power operation suffers from an increased circuit size. A technique to achieve a gamma conversion and inverse-gamma conversion by using an LUT (lookup table) may reduce the circuit size, compared with a technique using a circuit performing a power operation; however an LUT also has a relatively large circuit size and this approach does not provide a sufficient solution against the problem of the increase in the circuit size. The problem of the increase in the circuit size is especially serious in color adjustment in applications strongly requesting circuit size reduction, for example, in a display driver driving a display panel (e.g. a liquid crystal display panel and an OLED (organic light emitting diode) mounted on a mobile terminal).
As thus discussed, there is a technical need for achieving color adjustment on the basis of the gamma property of a display apparatus with a reduced circuit size.
It should be noted that International Publication No. WO2004/070699A discloses a technique that involves: dividing the color gamut of a display device into a plurality of regions with segments which connect the chromaticity coordinate points corresponding to the white color to those corresponding to the elementary color points and the complementary color points; determining which of the regions the chromaticity coordinate point corresponding to the input signal is positioned in; and correcting the RGB values of the input signal on the basis of suitable RGB correction values corresponding to the chromaticity coordinate points corresponding to the three vertices of the region in which the chromaticity coordinate point corresponding to the input signal is positioned.